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Traditionally, the first step in making cheese was to kill a newly-born, milk-fed calf and remove its stomach to make rennet. The rennet was derived from the inner lining of the abomasum, the fourth stomach of the calf or any other animal classified as a ruminant.

After scraping the stomach, the cheesemaker would dry it in the sun by stretching it on a rack. After the stomach was dry, it was cut it into squares or strips. Before the strips or squares were used, they were soaked in cold water and washed thoroughly before being placed in milk.

In an alternative process the strips or squares were dried, then ground, and finally mixed with a salt solution to extract rennin. Rennin is defined in Webster's Unabridged Dictionary as "a coagulating enzyme occurring in the gastric juice of the calf, forming the active principal of rennet and able to curdle milk." The cheese industry prefers a broader definition of rennin, calling it "any enzyme used for the controlled coagulation of milk."

What does rennin or rennet do?
Placed in milk, rennin or rennet breaks down a protein called kappa casein that keeps milk in liquid form. The breaking down of kappa casein leads to coagulation of the milk that will become cheese. Another term used for rennin is chymosin.

Not all cheese is made with animal-derived rennet. There are a number of rennetless cheeses whose coagulating enzymes are vegetable, microbial, or genetically engineered.
One group of rennetless cheeses has acidic levels high enough not to require enzymes for coagulation. This group includes cottage cheese, ricotta, and some varieties of mozzarella. Rennetless has also become a generic term for any cheese made without any animal derived enzymes.

Vegetable rennet usually means the enzyme was plant based. The phrase is an oxymoron because rennet implies it is animal derived. To add to the confusion, enzymes produced using microbes are often included in this category. What types of plants have been used to produce these enzymes? In the past, eager cheese makers have utilized plants like lady's bedstraw (Galium verum or curdwort), stinging nettle, fig leaves, melon, safflower, and wild thistle.

Microbial rennet can be produced by fermentation of the fungus Mucor miehe, Mucor Pusillus, and Endothia cryphonectria or from bacteria like Bacillus subililis or Bacillus prodigiosum. This type of rennet cannot be used to make cheddar or hard cheeses.

Genetically engineered rennet arose out of economic necessities. Supplies of animal rennet have always fluctuated and shortages have occurred. Supply problems have led to all types of research including one attempt in 1997 to create rennet from fish stomach mucosa, a waste product of the fishing industry. With bioengineered rennet the supply is always available and less expensive.

The bioengineering process involves taking a calf's prochymosin gene and inserting it into genomes of bacteria and yeast. In 1989 a microbial chymosin first appeared on the FDA's GRAS (Generally Regarded As Safe) list. In March 1990, after a 28-month review, the FDA approved a bioengineered form of rennin as the first genetically engineered product for human consumption.

According to information obtained from Whole Foods Market, "it is estimated that 70% of domestic cheese (in the United States) is produced with bioengineered chymosin. The producers of bioengineered rennet claim that their process will end the cheese industry's reliance on slaughtered calves.

Labeling
The problem for vegetarian consumers is determining whether a particular cheese contains animal ingredients. The FDA does not require that the cheese ingredient label denote the type of rennet. The FDA considers it impractical for cheese producers to have variations of labels to indicate what is included in enzyme mixtures. Cheese makers can mix animal, plant, and microbial varieties and just label them "enzymes."

Whole Foods, in a survey of one of its Texas markets, found eight different ways enzymes were listed on cheese packages. They reported the following: enzymes, microbial enzymes (non-animal, rennetless), rennetless, rennet, enzymes and rennet, vegetarian rennet, and microbial coagulants. They found a large portion of the labels just had the word "enzymes." A few of the labels did not list any coagulant. VIP's survey of local market chains found similar results with most cheeses listing the generic term "enzymes" or failing to list any coagulant. Imported cheeses either listed "rennet" or had no ingredient list.

Trader Joe's has produced a colorful pamphlet titled Rennet that is available in all stores. The publication briefly discusses the use of rennet in making cheese and provides information on the types of rennet. Most important, it classes the cheeses they sell by the type of coagulant used. As a bonus they tell which cheeses do not contain rBST (recombinant Bovine Somatotropin), a synthetic Bovine Growth Hormone.

The Solution
Navigating the minefield of cheese seems rather simple for the VIP editors. Eliminate the cheese. If it's made in Europe it's likely to have animal rennet but no rBST. If it is produced in the United States, it's likely to contain rBST and who knows what kind of coagulant. We always recommend that people read labels, but in this case the label may not provide complete or accurate information about the coagulant used.

The ethical dilemma for lacto vegetarians or lacto-ovo vegetarians is eating a product that was produced by killing an animal. Choosing a soy cheese may be a solution for the rennet dilemma, but soy or other types of vegetarian cheeses present their own problems. Many of these imitation cheeses contain casein, a milk protein. Any cheese labeled "vegan" should not contain animal-derived rennet or casein. However, it is important to read the label carefully because some manufacturers may not be aware that certain ingredients are animal-based.

Even though some vegetarian cheeses don't have the mouth feel and elasticity of rennet and casein cheeses, they're healthier and far more humane. Somehow, cutting a calf's stomach into pieces to create cheese, or inserting a calf gene into bacteria and yeast to produce bioengineered cheese is not very appealing. Next time, hold the cheese, please!